Optical light guides are well known for their use in applications requiring low-loss transmission of optical radiation through regions of heterogeneous media. In optical light guides, light incident on one end of a guide, within a range of allowed acceptance angles, is captured by the guide, transmitted through the guide, and emitted from the guide (opposite end) within the same range of allowed angles (entrance and exit faces being similar). A light guide is composed of a central core surrounded by a layer, commonly referred to as a cladding layer, whose refractive index is less than that of the core and whose function prevents light leakage from the guide during propagation. The numerical aperture (NA) of a light guide with discrete indices of refraction for both core (n1) and cladding layers (n2) is defined as the sine of the half maximum angle of acceptance (.sigma.) of light into the guide: EQU NA=sin .sigma.=(n1.sup.2 -n2.sup.2).sup.1/2
The larger the NA of a guide, the larger is its acceptance angle.
Optical fibers of high bandwidth, as used today by the communications industry to transmit light over great distances, are prime examples of optical light guides. Numerous applications for such guides exist, however, which do not require transmission of optical radiation over great distances. In the field of integrated optics, for example, non-fiber based light guides are used often to direct light over short distances (millimeters to centimeters) to active (e.g. electro-optic, acousto-optic, magneto-optic) elements for modulation, switching, filtering, signal processing, detection, etc. In areas of printing and image display, optical fibers of short length are used in a number of applications to guide light from one or more sources to a single plane of illumination. Fiber optic faceplates, i.e. collections of optical fibers, fused, cut normal to their length and polished into plates, are examples of elements used in these areas to guide light from one surface to another while preserving spatial information over two-dimensions.
As technology develops for capturing images of greater and greater resolution, corresponding technologies must also develop to enable the printing and display of such images. Print heads and image displays with an increased density of pixels, i.e., print or picture elements, will most certainly be required. Discrete sources of light coupled by optical fibers to an image or illumination plane will no longer be sufficient to satisfy needs and demand will grow for arrays of electro-optic emitters or optical light modulators on planar substrates formed via microlithography and thin film processing. Silicon is an attractive substrate for such devices in that it can incorporate much of the electronic circuitry required to control the devices. Silicon is also readily available in sizes which exceed 8" in diameter and many facilities exist with equipment dedicated for its processing. Silicon is not, however, transparent to light in the visible and ultraviolet regions of the optical spectrum. Light emitted from devices formed on silicon in one of these regions must, to be useful, propagate above the silicon surface. Often, however, due to the need for opaque encapsulents or opaque, top surface electrical contacts such light propagation cannot occur.